1. Field of the Invention
The present invention relates to an optically active polyester having photosensitivity; a photoreactive chiral agent which causes a change in liquid crystal structure; a liquid crystal composition, an optical film, a liquid crystal color filter and a recording medium containing this agent; and a method of changing or fixing the helical structure of a liquid crystal.
2. Description of the Related Art
In recent years, attention has been paid to liquid crystal materials. For example, a cholesteric liquid crystal compound, which has a helical structure and exhibits various selective reflected colors by the pitch of the helix, and the like compounds are superior in selective reflectivity and color purity in the range of the visible region. Therefore, these compounds are widely used as an optical film, a liquid crystal color filter, a recording medium and so on.
As a specific example thereof, a color filter will be described hereinafter.
For example, a color filter used in a color liquid crystal display or the like is generally composed of pixels in red (R), green (G) and blue (B), and a black matrix for improving display contrast, which is formed between the pixels. Conventionally, the main current of such a color filter is a filter comprising a pigment dispersed in a resin or a dyed colorant. The method of producing the color filter is in general a method of applying a color-resin solution to a glass substrate by spin coating or the like to form a colored resist layer, and patterning the layer by photolithography to form color filter pixels, or a method of printing colored pixels directly onto a substrate.
However, for example, the production method based on printing has drawbacks that color filters obtained by this method has a low resolution of pixels and does not cope easily with the formation of an highly-minute image pattern. The production method based on spin-coating has drawbacks that a loss of raw material is large and unevenness of coating is large in the case that this method is applied to a substrate having a large area. According to a production method based electrodeposition, it is possible to obtain a color filter having a relatively high resolution and a little unevenness of a colored layer. However, this method has drawbacks that producing steps therein are complicated and control of the solution is also difficult.
For the above-mentioned reasons, it has been demanded to develop, as a method of producing a color filter, a method which causes little loss in raw materials and which makes it possible to produce a color filter with high efficiency and ease.
As the performance of a color filter, high transmittance and high color purity are demanded. In recent years, the demands have been satisfied by optimizing, in the method using a colorant, the kind of the colorant or a resin to be dyed, or by making, in the method using a pigment, the pigment into particles dispersed into very fine particles. However, in recent liquid crystal display (LCD) panels, the transmittance and color purity of their color filter have been requested to be very high. Particularly in a reflection type color filter for LCD, it is difficult to satisfy all of white display corresponding to white paper, contrast, and color reproducibility. All color filters produced by dying a colorant in a resin or dispersing a pigment therein in the conventional production methods are light-absorbing type color filters; therefore, an improvement in color purity based on a more improvement in the transmittance thereof substantially reaches the upper limit.
In light of situation as described above, a color filter using polarized light, which is made mainly of cholesteric liquid crystal, is known. In the color filter using polarized light, light rays of a given quantity are reflected and light rays of a quantity other than the given quantity are transmitted to display images. Therefore, the color filter has a high efficiency of using light and is superior in transmittance and color purity to the light-absorbing type color filter. The production method thereof is in general a method of forming a film on a substrate by spin coating or the like in order to make the thickness of the film uniform. However, this method has a problem that a loss in raw materials is large and is disadvantageous from the viewpoint of costs.
As a method for solving the above-mentioned problems and making it possible to ensure the uniformity of the color purity or the like of a color filter film and realize a reduction in the number of the producing steps thereof, a method of using a photoreactive chiral compound is useful. This method is a principle as follows: when a liquid crystal composition containing a photoreactive chiral compound is irradiated patternwise with light having a reaction wavelength of the chiral compound, reaction of the chiral compound advances correspondingly to the intensity of irradiation energy so that the helical pitch (twisting angle) of the liquid crystal compound changes; therefore, selective reflected colors are formed in respective pixels only by pattern-exposure having a distribution in light energy. Thus, this method has an advantage that patterning at the time of forming a color filter is finished by carrying out mask-exposure using a mask having a distribution in transmitted light quantity only one time.
Accordingly, a film functioning as a color filter can be formed by irradiating a cholesteric liquid crystal compound imagewise with light to pattern the compound and then fixing the compound. This can be applied to an optical film, image-recording, and so on.
Particularly in the case that a color filter is produced by subjecting a liquid crystal to mask-exposure one time, it is desired that three primary colors of blue (B), green (G) and red (R) can be produced with high color purity by the one-time exposure. However, when a rate of change in the twist of the liquid crystal is small, sufficient color purity cannot be obtained. Therefore, in order to display the three primary colors having high color purity by one-time exposure, it is necessary to use, as a photoreactive chiral compound, a chiral compound (chiral agent) having a high rate of change in twisting so as to change the twisting power of its helical structure largely. That is, by using a chiral compound having a high rate of change in twisting, the light quantity thereof is changed, whereby the range of selective reflected colors can be enlarged.
As such a chiral agent, the Applicant filed a Japanese Patent Application about a low molecular-weight photoreactive chiral agent having an isosorbide skeleton (Japanese Patent Application No. 2001-5740). Moreover, WO 00/34808 discloses a low molecular-weight photoreactive chiral agent having a benzylidene menthone skeleton.
In the case that a cholesteric liquid crystal compound is irradiated imagewise with light to pattern the compound and then the patterned compound is fixed, an image having a high resolution cannot be obtained if the pattern is not maintained during the time from the patterning to the fixation.
However, these chiral agents have a problem that images having good resolution cannot be obtained.
As described above, in the case of a cholesteric liquid crystal phase which has a photoreactivity capable of changing orientation structures of a liquid crystal (for example, the helical pitch (the twisting power, or the twisting angle of the helix) of a liquid crystal) by the quantity of irradiation with light and which contains, for example, a nematic liquid crystal compound, a photoreactive chiral agent capable of changing the helical pitch (twisting power) thereof largely has not yet been provided in the present situation. That is, there has not yet been provided a photoreactive chiral agent which has a wide range of light wavelengths which can be selectively reflected and gives a wide selective reflection range and which can display three primary colors (B, G and R) with high color purity and good resolution.
Objects of the present invention are to solve the above-mentioned problems in the prior art.
That is, a first object of the present invention is to provide a new optically active compound which has photosensitivity and can be isomerized by light so as to be changed in structure.
A second object of the present invention is to provide a photoreactive chiral agent which can control the orientation of a liquid crystal compound and has a high rate of change in the helical pitch (twisting power) (which may be referred to as a xe2x80x9crate of change in twistingxe2x80x9d hereinafter) of a liquid crystal by light. For example, in the case of a cholesteric liquid crystal phase, this object is to provide a photoreactive chiral agent which can attain various selective reflections including reflections of three primary colors (B, G and R), and can display the three primary colors with high color purity and good resolution.
A third object of the present invention is to provide a liquid crystal composition containing a photoreactive chiral agent which can change the helical pitch (twisting power) of a liquid crystal by light and which has a high rate of change in twisting; and being capable of controlling the orientation state of liquid crystal molecules largely and three-dimensionally by light so as to change optical properties. For example, in the case of a cholesteric liquid crystal phase, this object is to provide a liquid crystal composition which can exhibit various selective reflected colors including three primary colors (B, G and R) by irradiation with light, and can display the three primary colors with high color purity and good resolution.
A fourth object of the present invention is to provide a method of changing the helical structure of a liquid crystal, in which the helical pitch (twisting power) of the liquid crystal can be changed by irradiating a liquid crystal composition containing a photoreactive chiral agent having a high rate of change in twisting with light.
A fifth object of the present invention is to provide a method of fixing the helical structure of a liquid crystal, in which a liquid crystal composition containing a photoreactive chiral agent having a high rate of change in twisting is exposed imagewise to light and subsequently the helical structure can be fixed in the state that the patterned helical pitch is kept without being damaged. Particularly in the case that the phase of the liquid crystal is a cholesteric liquid crystal phase, this object is to provide a method of fixing the helical structure of the liquid crystal, in which the helical structure of the liquid crystal is fixed into a desired selective reflected color so that a color tone having high color purity can be obtained.
A sixth object of the present invention is to provide a liquid crystal color filter which contains a photoreactive chiral agent capable of changing the helical pitch (twisting power) of a liquid crystal largely by irradiation with light and which has high color purity.
A seventh object of the present invention is to provide a non-light-absorbing type optical film containing a photoreactive chiral agent capable of changing the twisting power of a liquid crystal largely by irradiation with light. For example, in the case of a cholesteric liquid crystal phase, this object is to provide an optical film having a wide selective reflection range and high color purity.
An eighth object of the present invention is to provide a recording medium which contains a photoreactive chiral agent capable of changing the twisting power of a liquid crystal largely by irradiation with light and which is capable of forming a vivid image by changing light quantity imagewise. For example, in the case that the phase of the liquid crystal is a cholesteric liquid crystal phase, this object is to provide a recording medium capable of forming an image made of selective reflected colors having various color phases and high color purity.
Means of the present invention for overcoming the problems are as follows.
A first aspect of the present invention provides an optically active polyester comprising: a constituent unit represented by the general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group; and a constituent unit represented by the following general formula (II): 
in which A represents a bivalent substituent.
A second aspect of the present invention provides an optically active polyester according to the first aspect, wherein, in the general formula (I), the substituents represented by Ar1 and Ar2 are the same as one another, and the substituents represented by X1 and X2 are the same as one another and are each a hydrogen atom or a cyano group.
A third aspect of the present invention provides an optically active polyester according to the first aspect, wherein, in the general formula (II), A represents a bivalent aromatic group.
A fourth aspect of the present invention provides an optically active polyester according to the first aspect, further comprising a constituent unit represented by the following general formula (III):
xe2x80x94Oxe2x80x94Bxe2x80x94Oxe2x80x94xe2x80x83xe2x80x83General Formula (III)
in which B represents a bivalent substituent.
A fifth aspect of the present invention provides an optically active polyester according to the first aspect, wherein the constituent unit represented by the general formula (I) comprises a content thereof relative to all constituent components of from 10 to 90% by mol.
A sixth aspect of the present invention provides an optically active polyester according to the first aspect, wherein the constituent unit represented by the general formula (I) comprises a diol component, the constituent unit represented by the general formula (II) comprises a dicarboxylic acid component, and a composition ratio of the diol component/the dicarboxylic acid component is in the range from 80/20 to 20/80.
A seventh aspect of the present invention provides an optically active polyester according to the first aspect, wherein the optically active polyester comprises a polystyrene-equivalent number-average molecular weight of from 2,000 to 100,000.
An eighth aspect of the present invention provides a photoreactive chiral agent comprising an optically active polyester which includes: a constituent unit represented by the following general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group; and a constituent unit represented by the following general formula (II): 
in which A represents a bivalent substituent.
A ninth aspect of the present invention provides a liquid crystal composition comprising a liquid crystal compound and at least one optically active polyester which includes: a constituent unit represented by the following general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group; and a constituent unit represented by the following general formula 
in which A represents a bivalent substituent.
A tenth aspect of the present invention provides a liquid crystal composition according to the ninth aspect, wherein the liquid crystal compound is a nematic liquid crystal compound.
An eleventh aspect of the present invention provides a liquid crystal composition according to the ninth aspect, wherein the content of the optically active polyester in the liquid crystal composition is from 0.1 to 30% by mass.
A twelfth aspect of the present invention provides a liquid crystal composition according to the ninth aspect, wherein the liquid crystal compound has at least one polymerizable group, the composition further comprising a photopolymerization initiator.
A thirteenth aspect of the present invention provides a liquid crystal composition according to the twelfth, which further comprises a polymerizable monomer.
A fourteenth aspect of the present invention provides a liquid crystal composition according to the twelfth aspect, wherein the optically active polyester and the photopolymerization initiator each comprises a photosensitivity wavelength range, and the photosensitivity wavelength ranges are substantially different from each other.
A fifteenth aspect of the present invention provides a method for changing helical structure of a liquid crystal, the method comprising the steps of: preparing a liquid crystal composition including a liquid crystal compound and at least one optically active polyester which includes a constituent unit represented by the following general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group, and a constituent unit represented by the following general formula (II): 
in which A represents a bivalent substituent; and irradiating the liquid crystal composition with light for changing the structure of the optically active polyester.
A sixteenth aspect of the present invention provides a method for fixing helical structure of a liquid crystal, the method comprising the steps of: preparing a liquid crystal composition including a liquid crystal compound, a photopolymerization initiator and at least one optically active polyester which includes a constituent unit represented by the following general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group, and a constituent unit represented by the following general formula (II): 
in which A represents a bivalent substituent; irradiating the liquid crystal composition with light in a photosensitivity wavelength range of the optically active polyester; and then, irradiating the liquid crystal composition with light in a photosensitivity wavelength range of the photopolymerization initiator for carrying out photopolymerization.
A seventeenth aspect of the present invention provides a liquid crystal color filter comprising a liquid crystal compound and at least one optically active polyester which includes: a constituent unit represented by the following general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group; and a constituent unit represented by the following general formula (II): 
in which A represents a bivalent substituent.
An eighteenth aspect of the present invention provides an optical film comprising a liquid crystal compound and at least one optically active polyester which includes: a constituent unit represented by the following general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group; and a constituent unit represented by the following general formula (II): 
in which A represents a bivalent substituent.
A nineteenth aspect of the present invention provides a recording medium comprising a liquid crystal compound and at least one optically active polyester which includes: a constituent unit represented by the following general formula (I): 
in which Ar1 and Ar2 each independently represents a bivalent aromatic group or a bivalent heteroaromatic group, and X1 and X2 each independently represents a hydrogen atom or an electron-withdrawing group; and a constituent unit represented by the following general formula 
in which A represents a bivalent substituent.